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COMP 202: Principles of Object-Oriented Programming II

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Home — Fall 2008

A Java package is a grouping of classes similar to the notion of a directory is a grouping of files. Packages are used to help avoid name clashes and to hide particular pieces of code from the clients. A package has a name, such as utility or java.lang.util. In general, a package name is a series of strings of alphanumeric characters (starting with an alphabetic character) and separated by periods. To make a java class part of a particular package, say scheme, you must add the declaration package scheme; to the very top of the class source file.

Also, you will need to put the file in the directory structure that mirrors the package name. For example, the java classes that belong to the package scheme should be in a directory also named scheme. If you don't do this, it will still compile, but it won't run correctly.

Exercises:

Download and unzip the following file to use in the exercises: listSource.zip

Add the package scheme; declaration to the top of AList.java. You need to create a subdirectory called schemeand move AList.java into it.
Now compile using
```
     javac scheme/AList.java
     
```
Note that you should always compile from your project's main directory. If you compile from within a package subdirectory, it doesn't find all the supporting definitions.
We can't run anything yet, because that's just a piece of the whole program.
Add the package scheme; declaration to the top of EmptyList.java and NEList.java, and move them into the scheme subdirectory. Do not make ListClient.java part of the package. TestLength.java does not have a package name, and it thus said to be in the no-name (or default) package.
Also, remove the public access from the EmptyList and NEList classes. By default, a class is "package-private", i.e., it is known within the package, but not from outside. If you try to compile TestLength.java now, you will get an error message. Try it to see what happens.

You need to add the statement import scheme.*; to the top of TestLength.java to indicate to the compiler that you are using all the public classes in that package. Try to compile it again.

You should see a few error messages saying that you can't use EmptyList.java and NEList.java because these classes are not public. This is because the TestLength class is not part of the list package. One way to resolve this problem by making TestLength part of the list package. A class of a package can access all the classes (public or "package-private") in the package. However this is not a good solution in general because a client may be using many classes from different packages, but not class can be part of more than one package. A better solution is to use a "factory". We will discuss factories in another lab. For now, just make EmptyList.java and NEList.java public again, and recompile TestLength.java. You should get no error. Try to run TestLengthnow.

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